In the forthcoming cellular system known as LTE, Long Term Evolution, the downlink transmissions, i.e. transmission to the users in a cell, will use so called OFDM modulation, Orthogonal Frequency Division Multiplex, while the uplink transmissions, i.e. transmission from the users in a cell, will use either OFDM or OFDM-like technologies, such as DFTS-OFDM, a transmission technology which allows for orthogonal multiple access in time as well as in frequency.
Transmissions to and from users in a cell are made to/from a controlling node of the cell, this node in LTE being known as the eNodeB, “evolved NodeB”. Users in an LTE system are sometimes referred to as UEs, “User Equipment”.
In order to preserve the orthogonality needed in an LTE system, transmissions from the UEs in a cell need to be time aligned when they arrive at the eNodeB, i.e. the transmissions from the UEs in the cell of the eNodeB need to arrive more or less simultaneously at the eNodeB.
Since the different UEs in a cell may be located at different distances from the eNodeB of the cell, the UEs need to initiate their transmissions at different points in time in order for their transmissions to arrive simultaneously at the eNodeB. For example, a UE which is at the cell edge needs to start its transmissions prior to a UE which is closer to the eNodeB.
The issue of when to start the transmissions in the different UEs in a cell can be handled by means of a so called “timing advance”, in other words an “offset” value in time at which a UE needs to start its transmissions relative to a nominal transmission time specified by the eNodeB.
The value of the timing advance for a UE can be determined by the eNodeB by means of measuring the arrival of uplink transmissions from the UE, and the eNodeB then transmits the timing advance value to the UE with regular updates, since the UE may move around in the cell.
If a UE does make any transmissions for a period of time, the timing advance needed by the UE becomes uncertain, for example due to possible movement away from the eNodeB of the UE. In order to avoid unaligned UE transmissions, there is therefore typically in an LTE system a timer in both the eNodeB and the UE, which determines when a UE falls “out of synchronization” in uplink. Thus, if a UE has not received a new timing advance command from its eNodeB during a specified period of time, the UE will consider itself out of synchronization.
A UE which is out of synch and needs to initiate communication with its eNodeB will avail itself of a procedure known as Random Access, a procedure which is used in a number of cases, such as, for example:                Resynchronization,        Incoming handover,        Scheduling request (for a UE that is not allocated any other resource for contacting the base station),        Initial access, for UEs in the LTE_IDLE or LTE_DETACHED states.        
One of the Random Access procedures defined for LTE systems is a so called contention based procedure, which can be described briefly as follows:
The UE starts the Random Access procedure by randomly selecting one of the preambles available for contention-based random access, and then transmits the selected random access preamble on the physical random access channel, PRACH, to the eNodeB.
The eNodeB acknowledges reception of the preamble by transmitting a response message, which includes a timing advance value update to be used in future transmissions from the UE. Following this, the eNodeB transmits a second message which is in part used to trigger the establishment of radio resource control, and in part to uniquely identify the UE.
A problem can arise during the Random Access procedure if more than one UE happens to select one and the same preamble and transmit at the same time, or rather, if the transmissions arrive simultaneously at the eNodeB. In such a case, the eNodeB will resolve the conflict by transmitting a so called contention resolution message, which informs the UEs of which one of them has “won” the contention based procedure, and may thus communicate with the eNodeB.
However, although the contention conflict will be resolved in the manner described above, a problem will remain: a subsequent message, which is transmitted by the UE, will be sent before the contention resolution message, and thus, a number of such messages may be sent by different UEs, all with the same resources and with the same so called demodulation reference signal.
Due to this, the eNodeB is likely to arrive at an inaccurate channel estimate, particularly if the signals from the different UEs are approximately of equal strength. With an inaccurate channel estimate, the likelihood of correctly decoding the second message from the different UEs will decrease significantly, which in turn can lead to a situation in which all of the UEs involved need to restart the random access procedure with a new preamble.